Cleaved silicon thin film with rough surface

ABSTRACT

A technique for forming a film of material ( 12 ) from a donor substrate ( 10 ). The technique has a step of introducing energetic particles ( 22 ) through a surface of a donor substrate ( 10 ) to a selected depth ( 20 ) underneath the surface, where the particles have a relatively high concentration to define a donor substrate material ( 12 ) above the selected depth. An energy source is directed to a selected region of the donor substrate to initiate a controlled cleaving action of the substrate ( 10 ) at the selected depth ( 20 ), whereupon the cleaving action provides an expanding cleave front to free the donor material from a remaining portion of the donor substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from the provisional patent applicationentitled A CONTROLLED CLEAVAGE PROCESS AND RESULTING DEVICE, filed May12, 1997 and assigned application Ser. No. 60/046,276, and is adivisional of application Ser. No. 09/026,034 filed Feb. 19, 1998 nowU.S. Pat No. 6,013,563, the disclosures of which are hereby incorporatedby reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to the manufacture of substrates. Moreparticularly, the invention provides a technique including a method anddevice for cleaving a substrate in the fabrication of asilicon-on-insulator substrate for semiconductor integrated circuits,for example. But it will be recognized that the invention has a widerrange of applicability; it can also be applied to other substrates formulti-layered integrated circuit devices, three-dimensional packaging ofintegrated semiconductor devices, photonic devices, piezoelectronicdevices, microelectromechanical systems (“MEMS”), sensors, actuators,solar cells, flat panel displays (e.g., LCD, AMLCD), biological andbiomedical devices, and the like.

Craftsmen or more properly crafts-people have been building usefularticles, tools, or devices using less useful materials for numerousyears. In some cases, articles are assembled by way of smaller elementsor building blocks. Alternatively, less useful articles are separatedinto smaller pieces to improve their utility. A common example of thesearticles to be separated include substrate structures, such as a glassplate, a diamond, a semiconductor substrate, and others.

These substrate structures are often cleaved or separated using avariety of techniques. In some cases, the substrates can be separatedusing a saw operation. The saw operation generally relies upon arotating blade or tool, which cuts through the substrate material toseparate the substrate material into two pieces. This technique,however, is often extremely “rough” and cannot generally be used forproviding precision separations in the substrate for the manufacture offine tools and assemblies. Additionally, the saw operation often hasdifficulty separating or cutting extremely hard and/or brittlematerials, such as diamond or glass.

Accordingly, techniques have been developed to separate these hardand/or brittle materials using cleaving approaches. In diamond cutting,for example, an intense directional thermal/mechanical impulse isdirected preferentially along a crystallographic plane of a diamondmaterial. This thermal/mechanical impulse generally causes a cleavefront to propagate along major crystallographic planes, where cleavingoccurs when an energy level from the thermal/mechanical impulse exceedsthe fracture energy level along the chosen crystallographic plane.

In glass cutting, a scribe line using a tool is often impressed in apreferred direction on the glass material, which is generally amorphousin character. The scribe line causes a higher stress area surroundingthe amorphous glass material. Mechanical force is placed on each side ofthe scribe line, which increases stress along the scribe line until theglass material fractures, preferably along the scribe line. Thisfracture completes the cleaving process of the glass, which can be usedin a variety of applications, including households.

Although the techniques described above are satisfactory, for the mostpart, as applied to cutting diamonds or household glass, they havesevere limitations in the fabrication of small complex structures orprecision workpieces. For instance, the above techniques are often“rough” and cannot be used with great precision in fabrication of smalland delicate machine tools, electronic devices, or the like.Additionally, the above techniques may be useful for separating onelarge plane of glass from another, but are often ineffective forsplitting off, shaving, or stripping a thin film of material from alarger substrate. Furthermore, the above techniques may often cause morethan one cleave front, which join along slightly different planes, whichis highly undesirable for precision cutting applications.

From the above, it is seen that a technique for separating a thin filmof material from a substrate which is cost effective and efficient isdesirable.

SUMMARY OF THE INVENTION

According to the present invention, an improved technique for removing athin film of material from a substrate using a controlled cleavingaction is provided. This technique allows an initiation of a cleavingprocess on a substrate using a single or multiple cleave region(s)through the use of controlled energy (e.g., spatial distribution) andselected conditions to allow an initiation of a cleave front(s) and toallow it to propagate through the substrate to remove a thin film ofmaterial from the substrate.

In a specific embodiment, the present invention provides a process forforming a film of material from a donor substrate using a controlledcleaving process. The process includes a step of introducing energeticparticles (e.g., charged or neutral molecules, atoms, or electronshaving sufficient kinetic energy) through a surface of a donor substrateto a selected depth underneath the surface, where the particles are at arelatively high concentration to define a thickness of donor substratematerial (e.g., thin film of detachable material) above the selecteddepth. To cleave the donor substrate material, the method providesenergy to a selected region of the donor substrate to initiate acontrolled cleaving action in the donor substrate, whereupon thecleaving action is made using a propagating cleave front(s) to free thedonor material from a remaining portion of the donor substrate.

In most of the embodiments, a cleave is initiated by subjecting thematerial with sufficient energy to fracture the material in one region,causing a cleave front, without uncontrolled shattering or cracking. Thecleave front formation energy (E_(c)) must often be made lower than thebulk material fracture energy (E_(mat)) at each region to avoidshattering or cracking the material. The directional energy impulsevector in diamond cutting or the scribe line in glass cutting are, forexample, the means in which the cleave energy is reduced to allow thecontrolled creation and propagation of a cleave front. The cleave frontis in itself a higher stress region and once created, its propagationrequires a lower energy to further cleave the material from this initialregion of fracture. The energy required to propagate the cleave front iscalled the cleave front propagation energy (E_(p)). The relationship canbe expressed as:

 E_(c)=E_(p)+[cleave front stress energy]

A controlled cleaving process is realized by reducing E_(p) along afavored direction(s) above all others and limiting the available energyto below the E_(p) of other undesired directions. In any cleave process,a better cleave surface finish occurs when the cleave process occursthrough only one expanding cleave front, although multiple cleave frontsdo work.

Numerous benefits are achieved over preexisting techniques using thepresent invention. In particular, the present invention uses controlledenergy and selected conditions to preferentially cleave a thin film ofmaterial from a donor substrate which includes multi-material sandwichedfilms. This cleaving process selectively removes the thin film ofmaterial from the substrate while preventing a possibility of damage tothe film or a remaining portion of the substrate. Accordingly, theremaining substrate portion can be re-used repeatedly for otherapplications.

Additionally, the present invention uses a relatively low temperatureduring the controlled cleaving process of the thin film to reducetemperature excursions of the separated film, donor substrate, ormulti-material films according to other embodiments. This lowertemperature approach allows for more material and process latitude suchas, for example, cleaving and bonding of materials having substantiallydifferent thermal expansion coefficients. In other embodiments, thepresent invention limits energy or stress in the substrate to a valuebelow a cleave initiation energy, which generally removes a possibilityof creating random cleave initiation sites or fronts. This reducescleave damage (e.g., pits, crystalline defects, breakage, cracks, steps,voids, excessive roughness) often caused in pre-existing techniques.Moreover, the present invention reduces damage caused by higher thannecessary stress or pressure effects and nucleation sites caused by theenergetic particles as compared to preexisting techniques.

The present invention achieves these benefits and others in the contextof known process technology. However, a further understanding of thenature and advantages of the present invention may be realized byreference to the latter portions of the specification and attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-11 are simplified diagrams illustrating a controlled cleavingtechnique according to embodiments of the present invention;

FIG. 12A is a simplified diagram illustrating a controlled cleavingtechnique using dynamic pressure embodied as a high-pressure jet offluid or gas to separate a thin film of material from a donor waferaccording to another embodiment of the present invention;

FIG. 12B is a simplified diagram illustrating a controlled cleavingtechnique using static pressure to separate a thin film of material froma donor wafer according to another embodiment of the present invention;

FIG. 13 is a simplified diagram illustrating the use of static fluidpressure to separate a thin film of material from a donor wafer; and

FIGS. 14-18 are simplified cross-sectional view diagrams illustrating amethod of forming a silicon-on-insulator substrate according to thepresent invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENT

The present invention provides a technique for removing a thin film ofmaterial from a substrate while preventing a possibility of damage tothe thin material film and/or a remaining portion of the substrate. Thethin film of material is attached to or can be attached to a targetsubstrate to form, for example, a silicon-on-insulator wafer. The thinfilm of material can also be used for a variety of other applications.The invention will be better understood by reference to the Figs. andthe descriptions below.

1. Controlled Cleaving Techniques

FIG. 1 is a simplified cross-sectional view diagram of a substrate 10according to the present invention. The diagram is merely anillustration and should not limit the scope of the claims herein. Asmerely an example, substrate 10 is a silicon wafer which includes amaterial region 12 to be removed, which is a thin relatively uniformfilm derived from the substrate material. The silicon wafer 10 includesa top surface 14, a bottom surface 16, and a thickness 18. Substrate 10also has a first side (side 1) and a second side (side 2) (which arealso referenced below in the Figs.). Material region 12 also includes athickness 20, within the thickness 18 of the silicon wafer. The presentinvention provides a novel technique for removing the material region 12using the following sequence of steps.

Selected energetic particles implant 22 through the top surface 14 ofthe silicon wafer to a selected depth 24, which defines the thickness 20of the material region 12, termed the “thin film” of material. A varietyof techniques can be used to implant the energetic particles into thesilicon wafer. These techniques include ion implantation using, forexample, beam line ion implantation equipment manufactured fromcompanies such as Applied Materials, Eaton Corporation, Varian, andothers. Alternatively, implantation occurs using a plasma immersion ionimplantation (“PIII”) technique. Examples of plasma immersion ionimplantation techniques are described in “Recent Applications of PlasmaImmersion Ion Implantation,” Paul K. Chu, Chung Chan, and Nathan W.Cheung, SEMICONDUCTOR INTERNATIONAL, pp. 165-172, June 1996, and “PlasmaImmersion Ion Implantation—A Fledgling Technique for SemiconductorProcessing,” , P. K. Chu, S. Qin, C. Chan, N. W. Cheung, and L. A.Larson, MATERIALS SCIENCE AND ENGINEERING REPORTS: A REVIEW JOURNAL, pp.207-280, Vol. R17, Nos. 6-7, (Nov. 30, 1996), which are both herebyincorporated by reference for all purposes. Of course, techniques useddepend upon the application.

Depending upon the application, smaller mass particles are generallyselected to reduce a possibility of damage to the material region 12.That is, smaller mass particles easily travel through the substratematerial to the selected depth without substantially damaging thematerial region that the particles traverse through. For example, thesmaller mass particles (or energetic particles) can be almost anycharged (e.g., positive or negative) and/or neutral atoms or molecules,or electrons, or the like. In a specific embodiment, the particles canbe neutral and/or charged particles including ions such as ions ofhydrogen and its isotopes, rare gas ions such as helium and itsisotopes, and neon. The particles can also be derived from compoundssuch as gases, e.g., hydrogen gas, water vapor, methane, and hydrogencompounds, and other light atomic mass particles. Alternatively, theparticles can be any combination of the above particles, and/or ionsand/or molecular species and/or atomic species. The particles generallyhave sufficient kinetic energy to penetrate through the surface to theselected depth underneath the surface.

Using hydrogen as the implanted species into the silicon wafer as anexample, the implantation process is performed using a specific set ofconditions. Implantation dose ranges from about 10¹⁵ to about 10¹⁸atoms/cm², and preferably the dose is greater than about 10¹⁶ atoms/cm².Implantation energy ranges from about 1 KeV to about 1 MeV, and isgenerally about 50 KeV. Implantation temperature ranges from about −200to about 600° C., and is preferably less than about 400° C. to prevent apossibility of a substantial quantity of hydrogen ions from diffusingout of the implanted silicon wafer and annealing the implanted damageand stress. The hydrogen ions can be selectively introduced into thesilicon wafer to the selected depth at an accuracy of about +/−0.03 to+/−0.05 microns. Of course, the type of ion used and process conditionsdepend upon the application.

Effectively, the implanted particles add stress or reduce fractureenergy along a plane parallel to the top surface of the substrate at theselected depth. The energies depend, in part, upon the implantationspecies and conditions. These particles reduce a fracture energy levelof the substrate at the selected depth. This allows for a controlledcleave along the implanted plane at the selected depth. Implantation canoccur under conditions such that the energy state of the substrate atall internal locations is insufficient to initiate a non-reversiblefracture (i.e., separation or cleaving) in the substrate material. Itshould be noted, however, that implantation does generally cause acertain amount of defects (e.g., micro-detects) in the substrate thatcan typically at least partially be repaired by subsequent heattreatment, e.g., thermal annealing or rapid thermal annealing.

FIG. 2 is a simplified energy diagram 200 along a cross-section of theimplanted substrate 10 according to the present invention. The diagramis merely an illustration and should not limit the scope of the claimsherein. The simplified diagram includes a vertical axis 201 thatrepresents an energy level (E) (or additional energy) to cause a cleavein the substrate. A horizontal axis 203 represents a depth or distancefrom the bottom of the wafer to the top of the wafer. After implantingparticles into the wafer, the substrate has an average cleave energyrepresented as E 205, which is the amount of energy needed to cleave thewafer along various cross-sectional regions along the wafer depth. Thecleave energy (E_(c)) is equal to the bulk material fracture energy(E_(mat)) in non-implanted regions. At the selected depth 20, energy(E_(cz)) 207 is lower since the implanted particles essentially break orweaken bonds in the crystalline structure (or increase stress caused bya presence of particles also contributing to lower energy (E_(cz)) 207of the substrate) to lower the amount of energy needed to cleave thesubstrate at the selected depth. The present invention takes advantageof the lower energy (or increased stress) at the selected depth tocleave the thin film in a controlled manner.

Substrates, however, are not generally free from defects or “weak”regions across the possible cleave front or selected depth z₀ after theimplantation process. In these cases, the cleave generally cannot becontrolled, since they are subject to random variations such as bulkmaterial non-uniformities, built-in stresses, defects, and the like.FIG. 3 is a simplified energy diagram 300 across a cleave front for theimplanted substrate 10 having these defects. The diagram 300 is merelyan illustration and should not limit the scope of the claims herein. Thediagram has a vertical axis 301 which represents additional energy (E)and a horizontal axis 303 which represents a distance from side 1 toside 2 of the substrate, that is, the horizontal axis represents regionsalong the cleave front of the substrate. As shown, the cleave front hastwo regions 305 and 307 represented as region 1 and region 2,respectively, which have cleave energies less than the average cleaveenergy (E_(cz)) 207 (possibly due to a higher concentration of defectsor the like). Accordingly, it is highly likely that the cleave processbegins at one or both of the above regions, since each region has alower cleave energy than surrounding regions.

An example of a cleave process for the substrate illustrated by theabove FIG. is described as follows with reference to FIG. 4. FIG. 4 is asimplified top-view diagram 400 of multiple cleave fronts 401, 403propagating through the implanted substrate. The cleave fronts originateat “weaker” regions in the cleave plane, which specifically includesregions 1 and 2. The cleave fronts originate and propagate randomly asshown by the arrows. A limitation with the use of random propagationamong multiple cleave fronts is the possibility of having differentcleave fronts join along slightly different planes or the possibility offorming cracks, which is described in more detail below.

FIG. 5 is a simplified cross-sectional view 500 of a film cleaved from awafer having multiple cleave fronts at, for example, regions 1 305 and 2307. This diagram is merely an illustration and should not limit thescope of the claims herein. As shown, the cleave from region 1 joinedwith the cleave from region 2 at region 3 309, which is defined alongslightly different planes, may initiate a secondary cleave or crack 311along the film. Depending upon the magnitude of the difference 313, thefilm may not be of sufficient quality for use in manufacture ofsubstrates for integrated circuits or other applications. A substratehaving crack 311 generally cannot be used for processing. Accordingly,it is generally undesirable to cleave a wafer using multiple fronts in arandom manner. An example of a technique which may form multiple cleavefronts in a random manner is described in U.S. Pat. No. 5,374,564, whichis in the name of Michel Bruel (“Bruel”), and assigned to Commissariat A1'Energie Atomique in France. Bruel generally describes a technique forcleaving an implanted wafer by global thermal treatment (i.e., thermallytreating the entire plane of the implant) using thermally activateddiffusion. Global thermal treatment of the substrate generally causes aninitiation of multiple cleave fronts which propagate independently. Ingeneral, Bruel discloses a technique for an “uncontrollable” cleavingaction by way of initiating and maintaining a cleaving action by aglobal thermal source, which may produce undesirable results. Theseundesirable results include potential problems such as an imperfectjoining of cleave fronts, an excessively rough surface finish on thesurface of the cleaved material since the energy level for maintainingthe cleave exceeds the amount required, and many others. The presentinvention overcomes the formation of random cleave fronts by acontrolled distribution or selective positioning of energy on theimplanted substrate.

FIG. 6 is a simplified cross-sectional view of an implanted substrate 10using selective positioning of cleave energy according to the presentinvention. This diagram is merely an illustration, and should not limitthe scope of the claims herein. The implanted wafer undergoes a step ofselective energy placement or positioning or targeting which provides acontrolled cleaving action of the material region 12 at the selecteddepth. The impulse or impulses are provided using energy sources.Examples of sources include, among others, a chemical source, amechanical source, an electrical source, and a thermal sink or source.The chemical source can include particles, fluids, gases, or liquids.These sources can also include a chemical reaction to increase stress inthe material region. The chemical source is introduced as flood,time-varying, spatially varying, or continuous. In other embodiments, amechanical source is derived from rotational, translational,compressional, expansional, or ultrasonic energies. The mechanicalsource can be introduced as flood, time-varying, spatially varying, orcontinuous. In further embodiments, the electrical source is selectedfrom an applied voltage or an applied electromagnetic field, which isintroduced as flood, time-varying, spatially varying, or continuous. Instill further embodiments, the thermal source or sink is selected fromradiation, convection, or conduction. This thermal source can beselected from, among others, a photon beam, a fluid jet, a liquid jet, agas jet, an electro/magnetic field, an electron beam, a thermoelectricheating, a furnace, and the like. The thermal sink can be selected froma fluid jet, a liquid jet, a gas jet, a cryogenic fluid, a super-cooledliquid, a thermoelectric cooling means, an electro/magnetic field, andothers. Similar to the previous embodiments, the thermal source isapplied as flood, time-varying, spatially varying, or continuous. Stillfurther, any of the above embodiments can be combined or even separated,depending upon the application. Of course, the type of source useddepends upon the application.

In a specific embodiment, the present invention provides acontrolledpropagating cleave. The controlled-propagating cleave usesmultiple successive impulses to initiate and perhaps propagate acleaving process 700, as illustrated by FIG. 7. This diagram is merelyan illustration, and should not limit the scope of the claims herein. Asshown, the impulse is directed at an edge of the substrate, whichpropagates a cleave front toward the center of the substrate to removethe material layer from the substrate. In this embodiment, a sourceapplies multiple pulses (i.e., pulse 1, 2, and 3) successively to thesubstrate. Pulse 1 701 is directed to an edge 703 of the substrate toinitiate the cleave action. Pulse 2 705 is also directed at the edge 707on one side of pulse 1 to expand the cleave front. Pulse 3 709 isdirected to an opposite edge 711 of pulse 1 along the expanding cleavefront to further remove the material layer from the substrate. Thecombination of these impulses or pulses provides a controlled cleavingaction 713 of the material layer from the substrate.

FIG. 8 is a simplified illustration of selected energies 800 from thepulses in the preceding embodiment for the controlled-propagatingcleave. This diagram is merely an illustration, and should not limit thescope of the claims herein. As shown, the pulse 1 has an energy levelwhich exceeds average cleaving energy (E), which is the necessary energyfor initiating the cleaving action. Pulses 2 and 3 are made using lowerenergy levels along the cleave front to maintain or sustain the cleavingaction. In a specific embodiment, the pulse is a laser pulse where animpinging beam heats a selected region of the substrate through a pulseand a thermal pulse gradient causes supplemental stresses which togetherexceed cleave formation or propagation energies, which create a singlecleave front. In preferred embodiments, the impinging beam heats andcauses a thermal pulse gradient simultaneously, which exceeds cleaveenergy formation or propagation energies. More preferably, the impingingbeam cools and causes a thermal pulse gradient simultaneously, whichexceeds cleave energy formation or propagation energies.

Optionally, a built-in energy state of the substrate or stress can beglobally raised toward the energy level necessary to initiate thecleaving action, but not enough to initiate the cleaving action beforedirecting the multiple successive impulses to the substrate according tothe present invention. The global energy state of the substrate can beraised or lowered using a variety of sources such as chemical,mechanical, thermal (sink or source), or electrical, alone or incombination. The chemical source can include a variety such asparticles, fluids, gases, or liquids. These sources can also includechemical reaction to increase stress in the material region. Thechemical source is introduced as flood, time-varying, spatially varying,or continuous. In other embodiments, a mechanical source is derived fromrotational, translational, compressional, expansional, or ultrasonicenergies. The mechanical source can be introduced as flood,time-varying, spatially varying, or continuous. In further embodiments,the electrical source is selected from an applied voltage or an appliedelectromagnetic field, which is introduced as flood, time-varying,spatially varying, or continuous. In still further embodiments, thethermal source or sink is selected from radiation, convection, orconduction. This thermal source can be selected from, among others, aphoton beam, a fluid jet, a liquid jet, a gas jet, an electro/magneticfield, an electron beam, a thermoelectric heating, and a furnace. Thethermal sink can be selected from a fluid jet, a liquid jet, a gas jet,a cryogenic fluid, a super-cooled liquid, a thermoelectric coolingmeans, an electrolmagnetic field, and others. Similar to the previousembodiments, the thermal source is applied as flood, time-varying,spatially varying, or continuous. Still further, any of the aboveembodiments can be combined or even separated, depending upon theapplication. Of course, the type of source used also depends upon theapplication. As noted, the global source increases a level of energy orstress in the material region without initiating a cleaving action inthe material region before providing energy to initiate the controlledcleaving action.

In a specific embodiment, an energy source elevates an energy level ofthe substrate cleave plane above its cleave front propagation energy butis insufficient to cause self-initiation of a cleave front. Inparticular, a thermal energy source or sink in the form of heat or lackof heat (e.g., cooling source) can be applied globally to the substrateto increase the energy state or stress level of the substrate withoutinitiating a cleave front. Alternatively, the energy source can beelectrical, chemical, or mechanical. A directed energy source providesan application of energy to a selected region of the substrate materialto initiate a cleave front which self-propagates through the implantedregion of the substrate until the thin film of material is removed. Avariety of techniques can be used to initiate the cleave action. Thesetechniques are described by way of the Figs. below.

FIG. 9 is a simplified illustration of an energy state 900 for acontrolled cleaving action using a single controlled source according toan aspect of the present invention. This diagram is merely anillustration, and should not limit the scope of the claims herein. Inthis embodiment, the energy level or state of the substrate is raisedusing a global energy source above the cleave front propagation energystate, but is lower than the energy state necessary to initiate thecleave front. To initiate the cleave front, an energy source such as alaser directs a beam in the form of a pulse at an edge of the substrateto initiate the cleaving action. Alternatively, the energy source can bea cooling fluid (e.g., liquid, gas) that directs a cooling medium in theform of a pulse at an edge of the substrate to initiate the cleavingaction. The global energy source maintains the cleaving action whichgenerally requires a lower energy level than the initiation energy.

An alternative aspect of the invention is illustrated by FIGS. 10 and11. FIG. 10 is a simplified illustration of an implanted substrate 1000undergoing rotational forces 1001, 1003. This diagram is merely anillustration, and should not limit the scope of the claims herein. Asshown, the substrate includes a top surface 1005, a bottom surface 1007,and an implanted region 1009 at a selected depth. An energy sourceincreases a global energy level of the substrate using a light beam orheat source to a level above the cleave front propagation energy state,but lower than the energy state necessary to initiate the cleave front.The substrate undergoes a rotational force turning clockwise 1001 on topsurface and a rotational force turning counter-clockwise 1003 on thebottom surface which creates stress at the implanted region 1009 toinitiate a cleave front. Alternatively, the top surface undergoes acounter-clockwise rotational force and the bottom surface undergoes aclockwise rotational force. Of course, the direction of the forcegenerally does not matter in this embodiment.

FIG. 11 is a simplified diagram of an energy state 1100 for thecontrolled cleaving action using the rotational force according to thepresent invention. This diagram is merely an illustration, and shouldnot limit the scope of the claims herein. As previously noted, theenergy level or state of the substrate is raised using a global energysource (e.g., thermal, beam) above the cleave front propagation energystate, but is lower than the energy state necessary to initiate thecleave front. To initiate the cleave front, a mechanical energy meanssuch as rotational force applied to the implanted region initiates thecleave front. In particular, rotational force applied to the implantedregion of the substrates creates zero stress at the center of thesubstrate and greatest at the periphery, essentially being proportionalto the radius. In this example, the central initiating pulse causes aradially expanding cleave front to cleave the substrate.

The removed material region provides a thin film of silicon material forprocessing. The silicon material possesses limited surface roughness anddesired planarity characteristics for use in a silicon-on-insulatorsubstrate. In certain embodiments, the surface roughness of the detachedfilm has features that are less than about 60 nm, or less than about 40nm, or less than about 20 nm. Accordingly, the present inventionprovides thin silicon films which can be smoother and more uniform thanpre-existing techniques.

In a specific embodiment, the energy source can be a fluid jet that ispressurized (e.g., compressional) according to an embodiment of thepresent invention. FIG. 12A shows a simplified cross-sectional viewdiagram of a fluid jet from a fluid nozzle 608 used to perform thecontrolled cleaving process according to an embodiment of the presentinvention. The fluid jet 607 (or liquid jet or gas jet) impinges on anedge region of substrate 10 to initiate the controlled cleaving process.The fluid jet from a compressed or pressurized fluid source is directedto a region at the selected depth 603 to cleave a thickness of materialregion 12 from substrate 10 using force, e.g., mechanical, chemical,thermal. As shown, the fluid jet separates substrate 10 into tworegions, including region 609 and region 611 that separate from eachother at selected depth 603. The fluid jet can also be adjusted toinitiate and maintain the controlled cleaving process to separatematerial 12 from substrate 10. Depending upon the application, the fluidjet can be adjusted in direction, location, and magnitude to achieve thedesired controlled cleaving process. The fluid jet can be a liquid jetor a gas jet or a combination of liquid and gas. The fluid jet canseparate a thin film from the substrate at ambient (i.e. room)temperature, but the substrate and/or jet can also be heated or cooledto facilitate the separation process.

In a preferred embodiment, the energy source can be a compressionalsource such as, for example, compressed fluid that is static. FIG. 12Bshows a simplified cross-sectional view diagram of a compressed fluidsource 607 according to an embodiment of the present invention. Thecompressed fluid source 607 (e.g., pressurized liquid, pressurized gas)is applied to a sealed chamber 621, which surrounds a periphery or edgeof the substrate 10. As shown, the chamber is enclosed by device 623,which is sealed by, for example, O-rings 625 or the like, and whichsurrounds the outer edge of the substrate. The chamber has a pressuremaintained at PC that is applied to the edge region of substrate 10 toinitiate the controlled cleaving process at the selected depth ofimplanted material. The outer surface or face of the substrate ismaintained at pressure PA which can be ambient pressure e.g., 1atmosphere or less. A pressure differential exists between the pressurein the chamber, which is higher, and the ambient pressure. The pressuredifference applies force to the implanted region at the selected depth603. The implanted region at the selected depth is structurally weakerthan surrounding regions, including any bonded regions. Force is appliedvia the pressure differential until the controlled cleaving process isinitiated. The controlled cleaving process separates the thickness ofmaterial 609 from substrate material 611 to split the thickness ofmaterial from the substrate material at the selected depth.Additionally, pressure PC forces material region 12 to separate by a“prying action” from substrate material 611. During the cleavingprocess, the pressure in the chamber can also be adjusted to initiateand maintain the controlled cleaving process to separate material 12from substrate 10. Depending upon the application, the pressure can beadjusted in magnitude to achieve the desired controlled cleavingprocess. The fluid pressure can be derived from a liquid or a gas or acombination of liquid and gas. Optionally, a mechanical force, as from apin or blade, may be applied to the edge of the implanted region toinitiate the cleaving process, which typically reduces the maximumpressure differential required between the chamber and the ambient.

In a preferred embodiment, the present invention is practiced attemperatures that are lower than those used by pre-existing techniques.In particular, the present invention does not require increasing theentire substrate temperature to initiate and sustain the cleaving actionas preexisting techniques. In some embodiments for silicon wafers andhydrogen implants, substrate temperature does not exceed about 400° C.during the cleaving process. Alternatively, substrate temperature doesnot exceed about 350° C. during the cleaving process. Alternatively,substrate temperature is kept substantially below implantingtemperatures via a thermal sink, e.g., cooling fluid, cryogenic fluid.Accordingly, the present invention reduces a possibility of unnecessarydamage from an excessive release of energy from random cleave fronts,which generally improves surface quality of a detached film(s) and/orthe substrate(s). Accordingly, the present invention provides resultingfilms on substrates at higher overall yields and quality.

The above embodiments are described in terms of cleaving a thin film ofmaterial from a substrate. The substrate, however, can be disposed on aworkpiece such as a stiffener or the like before the controlled cleavingprocess. The workpiece joins to a top surface or implanted surface ofthe substrate to provide structural support to the thin film of materialduring controlled cleaving processes. The workpiece can be joined to thesubstrate using a variety of bonding or joining techniques, e.g.,electrostatics, adhesives, interatomic. Some of these bonding techniquesare described herein. The workpiece can be made of a dielectric material(e.g., quartz, glass, sapphire, silicon nitride, silicon dioxide), aconductive material (silicon, silicon carbide, polysilicon, group III/Vmaterials, metal), and plastics (e.g., polyimide-based materials). Ofcourse, the type of workpiece used will depend upon the application.

Alternatively, the substrate having the film to be detached can betemporarily disposed on a transfer substrate such as a stiffener or thelike before the controlled cleaving process. The transfer substratejoins to a top surface or implanted surface of the substrate having thefilm to provide structural support to the thin film of material duringcontrolled cleaving processes. The transfer substrate can be temporarilyjoined to the substrate having the film using a variety of bonding orjoining techniques, e.g., electrostatics, adhesives, interatomic. Someof these bonding techniques are described herein. The transfer substratecan be made of a dielectric material (e.g., quartz, glass, sapphire,silicon nitride, silicon dioxide), a conductive material (silicon,silicon carbide, polysilicon, group III/V materials, metal), andplastics (e.g., polyimide-based materials). Of course, the type oftransfer substrate used will depend upon the application. Additionally,the transfer substrate can be used to remove the thin film of materialfrom the cleaved substrate after the controlled cleaving process.

2. Silicon-On-Insulator Process

A process for fabricating a silicon-on-insulator substrate according tothe present invention may be briefly outlined as follows:

(1) Provide a donor silicon wafer (which may be coated with a dielectricmaterial);

(2) Introduce particles into the silicon wafer to a selected depth todefine a thickness of silicon film;

(3) Provide a target substrate material (which may be coated with adielectric material);

(4) Bond the donor silicon wafer to the target substrate material byjoining the implanted face to the target substrate material;

(5) Increase global stress (or energy) of implanted region at selecteddepth without initiating a cleaving action (optional);

(6) Provide stress (or energy) to a selected region of the bondedsubstrates to initiate a controlled cleaving action at the selecteddepth;

(7) Provide additional energy to the bonded substrates to sustain thecontrolled cleaving action to free the thickness of silicon film fromthe silicon wafer (optional);

(8) Complete bonding of donor silicon wafer to the target substrate; and

(9) Polish a surface of the thickness of silicon film.

The above sequence of steps provides a step of initiating a controlledcleaving action using an energy applied to a selected region(s) of amulti-layered substrate structure to form a cleave front(s) according tothe present invention. This initiation step begins a cleaving process ina controlled manner by limiting the amount of energy applied to thesubstrate. Further propagation of the cleaving action can occur byproviding additional energy to selected regions of the substrate tosustain the cleaving action, or using the energy from the initiationstep to provide for further propagation of the cleaving action. Thissequence of steps is merely an example and should not limit the scope ofthe claims defined herein. Further details with regard to the abovesequence of steps are described in below in references to the Figs.

FIGS. 14-19 are simplified cross-sectional view diagrams of substratesundergoing a fabrication process for a silicon-on-insulator waferaccording to the present invention. The process begins by providing asemiconductor substrate similar to the silicon wafer 2100, as shown byFIG. 14. Substrate or donor includes a material region 2101 to beremoved, which is a thin relatively uniform film derived from thesubstrate material. The silicon wafer includes a top surface 2103, abottom surface 2105, and a thickness 2107. Material region also includesa thickness (z₀), within the thickness 2107 of the silicon wafer.Optionally, a dielectric layer 2102 (e.g., silicon nitride, siliconoxide, silicon oxynitride) overlies the top surface of the substrate.The present process provides a novel technique for removing the materialregion 2101 using the following sequence of steps for the fabrication ofa silicon-on-insulator wafer.

Selected energetic particles 2109 implant through the top surface of thesilicon wafer to a selected depth, which defines the thickness of thematerial region, termed the thin film of material. As shown, theparticles have a desired concentration 2111 at the selected depth (z₀).A variety of techniques can be used to implant the energetic particlesinto the silicon wafer. These techniques include ion implantation using,for example, beam line ion implantation equipment manufactured fromcompanies such as Applied Materials, Eaton Corporation, Varian, andothers. Alternatively, implantation occurs using a plasma immersion ionimplantation (“PIII”) technique. Of course, techniques used depend uponthe application.

Depending upon the application, smaller mass particles are generallyselected to reduce a possibility of damage to the material region. Thatis, smaller mass particles easily travel through the substrate materialto the selected depth without substantially damaging the material regionthat the particles traversed through. For example, the smaller massparticles (or energetic particles) can be almost any charged (e.g.,positive or negative) and/or neutral atoms or molecules, or electrons,or the like. In a specific embodiment, the particles can be neutraland/or charged particles including ions of hydrogen and its isotopes,rare gas ions such as helium and its isotopes, and neon. The particlescan also be derived from compounds such as gases, e.g., hydrogen gas,water vapor, methane, and other hydrogen compounds, and other lightatomic mass particles. Alternatively, the particles can be anycombination of the above particles, and/or ions and/or molecular speciesand/or atomic species.

The process uses a step of joining the implanted silicon wafer to aworkpiece 2200 or target wafer, as illustrated in FIG. 15. The workpiecemay also be a variety of other types of substrates such as those made ofa dielectric material (e.g., quartz, glass, silicon nitride, silicondioxide), a conductive material (silicon, polysilicon, group III/Vmaterials, metal), and plastics (e.g., polyimide-based materials). Inthe present example, however, the workpiece is a silicon wafer.

In a specific embodiment, the silicon wafers are joined or fusedtogether using a low temperature thermal step. The low temperaturethermal process generally ensures that the implanted particles do notplace excessive stress on the material region, which can produce anuncontrolled cleave action. In one aspect, the low temperature bondingprocess occurs by a self-bonding process. In particular, one wafer isstripped to remove oxidation therefrom (or one wafer is not oxidized). Acleaning solution treats the surface of the wafer to form O—H bonds onthe wafer surface. An example of a solution used to clean the wafer is amixture of H₂O₂—H₂SO₄. A dryer dries the wafer surfaces to remove anyresidual liquids or particles from the wafer surfaces. Self-bondingoccurs by placing a face of the cleaned wafer against the face of anoxidized wafer.

Alternatively, a self-bonding process occurs by activating one of thewafer surfaces to be bonded by plasma cleaning. In particular, plasmacleaning activates the wafer surface using a plasma derived from gasessuch as argon, ammonia, neon, water vapor, and oxygen. The activatedwafer surface 2203 is placed against a face of the other wafer, whichhas a coat of oxidation 2205 thereon. The wafers are in a sandwichedstructure having exposed wafer faces. A selected amount of pressure isplaced on each exposed face of the wafers to self-bond one wafer to theother.

Alternatively, an adhesive disposed on the wafer surfaces is used tobond one wafer onto the other. The adhesive includes an epoxy,polyimide-type materials, and the like. Spin-on-glass layers can be usedto bond one wafer surface onto the face of another. These spin-on-glass(“SOG”) materials include, among others, siloxanes or silicates, whichare often mixed with alcohol-based solvents or the like. SOG can be adesirable material because of the low temperatures (e.g., 150 to 250°C.) often needed to cure the SOG after it is applied to surfaces of thewafers.

Alternatively, a variety of other low temperature techniques can be usedto join the donor wafer to the target wafer. For instance, anelectro-static bonding technique can be used to join the two waferstogether. In particular, one or both wafer surface(s) is charged toattract to the other wafer surface. Additionally, the donor wafer can befused to the target wafer using a variety of commonly known techniques.Of course, the technique used depends upon the application.

After bonding the wafers into a sandwiched structure 2300, as shown inFIG. 16, the method includes a controlled cleaving action to remove thesubstrate material to provide a thin film of substrate material 2101overlying an insulator 2305 the target silicon wafer 2201. Thecontrolled-cleaving occurs by way of selective energy placement orpositioning or targeting 2301, 2303 of energy sources onto the donorand/or target wafers. For instance, an energy impulse(s) can be used toinitiate the cleaving action. The impulse (or impulses) is providedusing an energy source which include, among others, a mechanical source,a chemical source, a thermal sink or source, and an electrical source.

The controlled cleaving action is initiated by way of any of thepreviously noted techniques and others and is illustrated by way of FIG.16. For instance, a process for initiating the controlled cleavingaction uses a step of providing energy 2301, 2303 to a selected regionof the substrate to initiate a controlled cleaving action at theselected depth (z₀) in the substrate, whereupon the cleaving action ismade using a propagating cleave front to free a portion of the substratematerial to be removed from the substrate. In a specific embodiment, themethod uses a single impulse to begin the cleaving action, as previouslynoted. Alternatively, the method uses an initiation impulse, which isfollowed by another impulse or successive impulses to selected regionsof the substrate. Alternatively, the method provides an impulse toinitiate a cleaving action which is sustained by a scanned energy alongthe substrate. Alternatively, energy can be scanned across selectedregions of the substrate to initiate and/or sustain the controlledcleaving action.

Optionally, an energy or stress of the substrate material is increasedtoward an energy level necessary to initiate the cleaving action, butnot enough to initiate the cleaving action before directing an impulseor multiple successive impulses to the substrate according to thepresent invention. The global energy state of the substrate can beraised or lowered using a variety of sources such as chemical,mechanical, thermal (sink or source), or electrical, alone or incombination. The chemical source can include particles, fluids, gases,or liquids. These sources can also include chemical reaction to increasestress in the material region. The chemical source is introduced asflood, time-varying, spatially varying, or continuous. In otherembodiments, a mechanical source is derived from rotational,translational, compressional, expansional, or ultrasonic energies. Themechanical source can be introduced as flood, time-varying, spatiallyvarying, or continuous. In further embodiments, the electrical source isselected from an applied voltage or an applied electromagnetic field,which is introduced as flood, time-varying, spatially varying, orcontinuous. In still further embodiments, the thermal source or sink isselected from radiation, convection, or conduction. This thermal sourcecan be selected from, among others, a photon beam, a fluid jet, a liquidjet, a gas jet, an electrolmagnetic field, an electron beam, athermoelectric heating, and a furnace. The thermal sink can be selectedfrom a fluid jet, a liquid jet, a gas jet, a cryogenic fluid, asuper-cooled liquid, a thermoelectric cooling means, an electro/magneticfield, and others. Similar to the previous embodiments, the thermalsource is applied as flood, time-varying, spatially varying, orcontinuous. Still further, any of the above embodiments can be combinedor even separated, depending upon the application. Of course, the typeof source used depends upon the application. As noted, the global sourceincreases a level of energy or stress in the material region withoutinitiating a cleaving action in the material region before providingenergy to initiate the controlled cleaving action.

In a preferred embodiment, the method maintains a temperature which isbelow a temperature of introducing the particles into the substrate. Insome embodiments, the substrate temperature is maintained between −200and 450° C. during the step of introducing energy to initiatepropagation of the cleaving action. Substrate temperature can also bemaintained at a temperature below 400° C. or below 350° C. In preferredembodiments, the method uses a thermal sink to initiate and maintain thecleaving action, which occurs at conditions significantly below roomtemperature.

A final bonding step occurs between the target wafer and thin film ofmaterial region according to some embodiments, as illustrated by FIG.17. In one embodiment, one silicon wafer has an overlying layer ofsilicon dioxide, which is thermally grown overlying the face beforecleaning the thin film of material, as shown in FIG. 15. The silicondioxide can also be formed using a variety of other techniques, e.g.,chemical vapor deposition. The silicon dioxide between the wafersurfaces fuses together thermally in this process.

In some embodiments, the oxidized silicon surface from either the targetwafer or the thin film of material region (from the donor wafer) arefurther pressed together and are subjected to an oxidizing ambient 2401.The oxidizing ambient can be in a diffusion furnace for steam oxidation,hydrogen oxidation, or the like. A combination of the pressure and theoxidizing ambient fuses the thin film of silicon material 2101 to thetarget silicon wafer 2201 together at the oxide surface or interface2305. These embodiments often require high temperatures (e.g., 700° C.).

Alternatively, the two silicon surfaces are further pressed together andsubjected to an applied voltage between the two wafers. The appliedvoltage raises temperature of the wafers to induce a bonding between thewafers. This technique limits the amount of crystal defects introducedinto the silicon wafers during the bonding process, since substantiallyno significant mechanical force is needed to initiate the bonding actionbetween the wafers. Of course, the technique used depends upon theapplication.

After bonding the wafers, silicon-on-insulator has a target substratewith an overlying film of silicon material and a sandwiched oxide layerbetween the target substrate and the silicon film, as also illustratedin FIG. 15 The detached surface of the film of silicon material is oftenrough 2404 and needs finishing. Finishing occurs using a combination ofgrinding and/or polishing techniques. In some embodiments, the detachedsurface undergoes a step of grinding using, for examples, techniquessuch as rotating an abrasive material overlying the detached surface toremove any imperfections or surface roughness therefrom. A machine suchas a “back grinder” made by a company called Disco may provide thistechnique.

Alternatively, chemical mechanical polishing or planarization (“CMP”)techniques finish the detached surface of the film, as illustrated byFIG. 18. In CMP, a slurry mixture is applied directly to a polishingsurface 2501 which is attached to a rotating platen 2503. This slurrymixture can be transferred to the polishing surface by way of anorifice, which is coupled to a slurry source. The slurry is often asolution containing an abrasive and an oxidizer, e.g., H₂O₂, KIO₃,ferric nitrate. The abrasive is often a borosilicate glass, titaniumdioxide, titanium nitride, aluminum oxide, aluminum trioxide, ironnitrate, cerium oxide, silicon dioxide (colloidal silica), siliconnitride, silicon carbide, graphite, diamond, and any mixtures thereof.This abrasive is mixed in a solution of deionized water and oxidizer orthe like. Preferably, the solution is acidic.

This acid solution generally interacts with the silicon material fromthe wafer during the polishing process. The polishing process preferablyuses a poly-urethane polishing pad. An example of this polishing pad isone made by Rodel and sold under the trade name of IC-1000. Thepolishing pad is rotated at a selected speed. A carrier head which picksup the target wafer having the film applies a selected amount ofpressure on the backside of the target wafer such that a selected forceis applied to the film. The polishing process removes about a selectedamount of film material, which provides a relatively smooth film surface2601 for subsequent processing, as illustrated by FIG. 18.

In certain embodiments, a thin film of oxide 2406 overlies the film ofmaterial overlying the target wafer, as illustrated in FIG. 17. Theoxide layer forms during the thermal annealing step, which is describedabove for permanently bonding the film of material to the target wafer.In these embodiments, the finishing process is selectively adjusted tofirst remove oxide and the film is subsequently polished to complete theprocess. Of course, the sequence of steps depends upon the particularapplication.

Although the above description is in terms of a silicon wafer, othersubstrates may also be used. For example, the substrate can be almostany monocrystalline, polycrystalline, or even amorphous type substrate.Additionally, the substrate can be made of III/V materials such asgallium arsenide, gallium nitride (GaN), and others. The multi-layeredsubstrate can also be used according to the present invention. Themulti-layered substrate includes a silicon-on-insulator substrate, avariety of sandwiched layers on a semiconductor substrate, and numerousother types of substrates. Additionally, the embodiments above weregenerally in terms of providing a pulse of energy to initiate acontrolled cleaving action. The pulse can be replaced by energy that isscanned across a selected region of the substrate to initiate thecontrolled cleaving action. Energy can also be scanned across selectedregions of the substrate to sustain or maintain the controlled cleavingaction. One of ordinary skill in the art would easily recognize avariety of alternatives, modifications, and variations, which can beused according to the present invention.

While the above is a full description of the specific embodiments,various modifications, alternative constructions and equivalents may beused. Therefore, the above description and illustrations should not betaken as limiting the scope of the present invention which is defined bythe appended claims.

What is claimed is:
 1. A device comprising a thin film of silicon, thethin film of silicon having a cleaved surface with a cleaved surfaceroughness less than about 60 nm, the cleaved surface being separatedfrom a substrate by a propagating cleave front.
 2. The device of claim 1wherein the thin film of silicon is less than about 15 microns thick. 3.The device of claim 1 wherein the thin film of silicon is bonded to atarget wafer.
 4. A device comprising a thin film of silicon, the thinfilm of silicon having a first surface, the first surface being aportion of a release layer, the release layer being disposed beneath asurface of a silicon substrate to divide the silicon substrate into thethin film of silicon and a remaining portion of the silicon substrate,the thin film of silicon being separated from the silicon substrate by apropagating cleave front.
 5. The device of claim 4 wherein thepropagating cleave front is initiated by the application of an amount ofenergy to a first region of the silicon substrate.
 6. The device ofclaim 4 wherein the thin film of silicon is bonded to a target wafer.